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I'm after some advice on substrate for my tank. I've got a hardscape wall running from one side to the other, towards the back, and there won't be any planting in the foreground - just a thin layer of sand and some scatter gravel.

Behind the wall is where the majority of my stem plants will be - Vallisneria, Amazon swords, that sort of thing. I want to add some immediate height behind the wall, so I'm looking to bulk up the substrate. I'll be using some form of aquasoil for nutrients, and probably crushed up root tabs.

I know the ideal solution would be to just use aquasoil all the way, but that's a bit pricey and I don't think it's necessary. I've come up with a couple of options:

1. Aquasoil in mesh bags, scatter some crushed up root tabs, and cap with the cheap sand I'll be using in the foreground.
Pros: it's cheap, and the substrate will look the same from the side.
Cons: I'm not sure about using sand for plant growth, and there's a risk of ammonia pockets, especially with so much sand. Plus, it'll be hard to disturb and churn when vacuuming, given how heavily planted the back will be.

2. Cheap black gravel topped with aquasoil - or maybe the other way round, I'm not sure.
Pros: it's cheap, and the gravel will allow for good flow, so no nasty pockets. Plants grow well in it too.
Cons: the substrate will look different from the side - black at the back, sand at the front. And I'm not sure if the gravel and aquasoil will mix and cause any problems.

Any thoughts on these options?

I've come across a similar setup where they've used all aquasoil behind the hardscape wall. I'm not sure if it's worth the investment, as it does seem to achieve the desired height. I'm still leaning towards one of my initial ideas, but I thought it was worth sharing this example.

Considering your hardscape wall and the desire for height, I think using plastic containers or rocks could be a viable option to bulk up the substrate behind it. For instance, you could use plastic containers with lids to fill in the space, and if you plan on keeping loaches or catfish, you could even make a hole in the container and place it between the rocks to create a little cave for them. Then, you can just cover the container with your chosen substrate. Alternatively, you could also use rocks to achieve a similar effect.

I've been thinking about what you could use under your soil layer, and I reckon lava rock chips would be a good choice. They're basically small chunks, about 2-4cm in size, and you can get them cheaply from garden centres or places that sell BBQ stuff. I saw a big sack of them once for £15, which is probably twice the size of those big bags of aquarium soil or sand. You could either put them in bags or just add soil on top and let it fill in the gaps.

I've been doing some further research and it seems a lot of aquasoils release a fair amount of ammonia. I'm just wondering if this would still be the case if I were to use the mesh bag method I mentioned earlier, with the aquasoil in the bags and then topped with sand.

Yeah, I've found that even with sand on top, the ammonia still manages to leach up, just at a slower pace.

I've been reading that ammonia leaching is a common issue with aquasoils - is this specific to certain brands or a characteristic of all aquasoils in general. I'd also appreciate any insight into how long this process can take, as I'm planning on doing a fish-in cycle using established filter material.

Most brands do leach ammonia, and it's not just a one-time thing - it can go on for a few weeks. I've found that one brand stands out from the rest, though - Wio Eonian. I've switched to using it and so far, I've been really happy with the results.